Let us Understand About Focal Length
When parallel rays of light coming in from a distant source hit a convex lens they undergo a phenomenon called refraction wherein they bend inward and converge to meet at a specific point. The point where the rays of light converge is called the focal point of the lens and the plane in which they converge is called the focal plane. The distance between the centre of the lens and the focal point is called the focal length of the lens. This is illustrated in the diagram below.
Now, let’s try and understand how focal length of a lens is measured.
Here is an illustration of how the rays of light converge in a lens.
The various glass elements of the lens converge the rays of light coming in from an external source and they converge at the focal point of the lens. They then intersect and fall on the camera sensor to form an image. Since the rays of light intersect each other before they form the image on the camera sensor, the image formed on the sensor is inverted. The focal length of the lens is the distance between the point of convergence and the camera sensor.
So, when we say the focal length of a lens is 50mm it means that the rays of light converge at a distance of 50mm from the camera sensor. In case of a zoom lens having a focal range of 100-400mm; it means that the convergence point is 100mm from the sensor when the lens is not zoomed and the distance of the convergence point increases to 400mm from the sensor when the lens is fully zoomed.
